There's no requirement to drinking a gallon of water per day to stay healthy.
However if you want to drink a gallon of water everyday to stay healthy you certainly can as 1 gallon of water per day will keep your kidneys flushed and help prevent bladder infections, kidney disease, kidney stones etc.
Always drink water or other hydrating drinks before you get thirsty to prevent dehydration.
As long as you're drinking 1 or 2 glasses of water per day or a few bottles of water per day and drinking other healthy drinks you'll be okay.
To stay healthy you're supposed to drink at least 8 glasses of water or even 1 gallon of water per day.
However you don't have to drink 1 gallon of water everyday or even 8 glasses of water everyday.
As long as you're drinking 1 or 2 glasses of water or a few bottles of water per day you'll still keep healthy.